Ventilation-Perfusion Pt 1

Question 1

What is the alveolar gas equation?

Answer 1

[Oxygen]alveolar = [Oxygen in lungs] - [Oxygen used by tissue]

PAo2 = PIo2 - PaCO2/R

PAo2 = alveolar partial pressure of oxygen
PIo2 - inspired partial pressure of oxygen
PaCO2 = arterial CO@
R= VdotCO2/VdotO2 = 0.8

PiO2 = (760-47) x oxygen % (to account for water vapor)

Question 2

What does the alveolar gas exchange equation calculate?

Answer 2

It calculates what inspired O2 needs to be to produce a desired alveolar and therefore arterial O2 level

Question 3

How is Inspired Partial Pressure of Oxygen (PIO2) calculated?

Answer 3

= (Barometric P - Water vapor P) x FiO2

BP = 760 mm Hg
WVP= 46 mm Hg
FiO2 (fraction of inspired oxygen) = 21% (0.21)

Question 4

What does the PAO2 allow us to calculate?

Answer 4

It is not very useful on it’s own

But can be used to calculate to A-a O2 Gradient - which is useful in determining the health of the alveoli

Question 5

What is the equation for A-a O2 gradient?

Answer 5

= PAO2 - PaO2

Normally < 20 mm Hg

Question 6

What does an increase in the A-a O2 gradient indicate?

Answer 6

Diffusion impairment

something is wrong with the alveoli

Question 7

What is the intrapleural fluid and pressure like in the apex of the lung?

Answer 7

Intrapleural fluid is acted on by gravity and affects the pressure

At the apex, there is less intrapleural fluid, so the intrapleural pressure is more negative (up to -10 cm H2O)

Question 8

What is the intrapleural fluid and pressure like in the base of the lung?

Answer 8

Intrapleural fluid is acted on by gravity and affects the pressure

At the apex, there is more intrapleural fluid, so the intrapleural pressure is less negative (-2 - 3 cm H2O)

Question 9

How does the intrapleural pressure affect the alveoli?

Answer 9

At the apex, the pressure is more negative so the alveoli are LARGE at rest

At the base, the pressure is less negative, so alveoli are SMALL at rest

Question 10

What is pressure and fluid like in the middle of the lung (Zone 2)?

Answer 10

It is “normal”

Just like with blood flow

Question 11

How do the alveoli in the apex of the lungs react to inspiration?

Answer 11

They can only get a little larger since already so large

Question 12

How do the alveoli in the base of the lungs react to inspiration?

Answer 12

The alveoli get much larger

Question 13

What is V/Q?

Answer 13

The mathematical ratio between ventilation and perfusion

Question 14

What is the average V/Q?

Answer 14

4/5 = 0.8

Question 15

What is High V/Q?

Answer 15

Ventilation is high relative to perfusion

Question 16

What is Low V/Q?

Answer 16

Ventilation is low compared to perfusion

Question 17

What can cause Low V/Q?

What happens?

What is this called?

Answer 17

Extreme condition V=0

The airway is blocked by something

But the blood continues to arrive at the alveolus

In a short time the O2 in the alveolus decreases because the blood has removed it all

While the CO2 increases (the blood continues to deliver CO2)

The same changed occur in the arterial blood

With time the alveolus will equilibrate with the non-arterialized (venous) blood entering the pulmonary capillary and the blood leaving that alveolus will leave without exchanged oxygen or carbon dioxide

*This represents a shunt - based on O2 and CO2 - the blood gained nothing by going to the lungs = as if the blood never went to the lungs

Question 18

What are the arterial blood gases that are attained by regions with low V/Q ratio?

Answer 18

PaO2: Low
PaCO2: High
pHa: Low
Volume of Blood: High relative to V.

Question 19

What is a high V/Q ratio mean?

Answer 19

Extreme: No blood coming to the alveolus (alveolar dead space)

With less blood removing oxygen and adding CO2, the alveolar O2 will increase and CO2 decrease, bringing the alveolar closer to atmospheric conditions

Eventually the alveolar air in these areas will be most similar to the air outside of the body

Not much blood benefits from this since not substantial blood flow

Even worse, the blood that couldn’t get to this alveolus has to go somewhere else. It goes to the normal alveolus and turns it into a low V/Q alveolus. Because even though the airflow is normal, the blood flow is increased, so the V/Q ratio decreases.

Overall PaO2 and PaCO2 decreases

Question 20

What are the arterial blood gases attained by regions with high V/Q ratio?

Answer 20

PaO2: High
PaCO2: Low
pHa: High
Volume of blood: Low

Question 21

What is the V/Q ratio at the apex of the lung?

Answer 21

HIGH

Leading to higher PaO2 and lower PaCO2

Question 22

What is the V/Q ratio at the middle of the lung? Zone 2

Answer 22

Normal

Question 23

What is the V/Q ratio at the base of the lung?

Answer 23

LOW

Blood gases are lower

Question 24

How can we minimize differences in V/Q ratios in different zones of the lung?

Answer 24

Hypoxic Vasocontriction

- contraction of pulmonary vessels to redirect blood away from hypoxic regions